Polyimide resins are superior in heat resistance, mechanical properties and electrical characteristics and thus, have been used widely as electric and electronic parts and aerospace materials. Because of their superior chemical resistance, polyimide resins are less soluble in solvent and thus, polyamic acids, precursors of polyimides, have been widely used. For preparation of final products such as film, a solution of polyamic acid should be applied and then imidated, for example, by heat or chemistry treatment. However, the polyamic acid is accompanied with solvent vaporization and dehydration during imidation, i.e., during film formation, leaving shrinkage stress in the resulting film and possibly causing problems of deformation and warp of the film.
Proposed as the means to solve the problems is a method of blending a polyamic acid with a polyamide-imide (see Patent Documents 1 and 2). For example in the case of Patent Document 1, the polyamide-imide should be blocked at the terminal and the insulation film obtained from a mixture of polyamide-imide and polyamic acid had a sea-island structure, showing that the components therein were not compatible with each other. Disadvantageously for that reason, the final product from such a mixture was less transparent. Alternatively in the case of Patent Document 2, the polyamide-imide and the polyamic acid were compatible with each other and gave a transparent film, but the method had a problem that the raw material naphthalene diisocyanate, which gives the component structure essential for the polyamide-imide, is expensive and highly toxic, and thus the product is restricted in its flexibility of use.    Patent Document 1: Japanese Patent application Laid-Open No. 2008-016266    Patent Document 2: Japanese Patent application Laid-Open No. 2005-325329